1. Field of Invention
The present invention relates to an improved method for calibrating optical spectrographs, and in particular optical spectrographs having focal plane array detectors.
2. Background
A spectrograph spatially disperses light focused at an entrance slit onto an exit focal plane, where it may be detected by a detector. A spectrograph is calibrated when the spatial position of the various wavelength components of the dispersed light at the exit focal plane is known with some degree of accuracy. A spectrograph with a focal plane array detector may be calibrated by placing a light source with a plurality of known emission wavelengths in front of the entrance slit, rotating the grating to some angle and noting the position(s) in the array where the detector is illuminated.
Traditional calibration methods require substantial user input, assume sophistication on the user's part, and provide limited feedback on the accuracy of the calibration across the spectral range. In addition, the accuracy of traditional routines is partially limited by the number of emission lines observed (herein referred to as observables) at a given grating angle as a result of quantifying the calibration in wavelength space, e.g. a direct wavelength to detector pixel coordinate correlation. Moreover, it is easy for a user to collect data with an uncalibrated instrument. In the best case, post-acquisition spectral calibration is tedious; in the worst case, the data are invalid and the experiment must be repeated. Patents and publications in this field include: U.S. Pat. Nos. 7,339,665; 6,876,448; 6,700,661, and 6,362,878; and U.S. Patent Application Pub. no. 2006/0290929. All references cited herein are incorporated by reference.
A need exists, therefore, for an improved calibration method for optical spectrographs.